The present invention relates to an accelerometer of a type having no proof or inertial mass and no moving parts or parts under stress such as piezo or strain gauge accelerometers.
Accelerometers find use in widely diverse applications including automobile air bags and suspension systems, computer hard disc drivers, smart detonation systems for bombs and missiles and machine vibration monitors. Silicon micromachined acceleration sensors are beginning to replace mechanical acceleration switches. Present accelerometers are all based upon the classical Newtonian relationship of force, F, mass, m, and acceleration, a, in which F=ma. Thus, for a cantilevered beam, the force due to acceleration causes the beam to deflect. This deflection is sensed either by sensing the change in piezo resistance or by a change in capacitance. Such systems are not stable over wide temperature ranges and have a response which peaks due to insufficient mechanical damping.
One form of accelerometer made by bulk micromachining consists of a membrane or diaphragm of silicon formed by chemical etching having a large mass of silicon at the centre and tethers of thin film piezoresistors, whose resistance is sensitive to strain and deformation, suspending the mass. Acceleration causes the large silicon mass to move, deforming the diaphragm and changing the resistance of the piezo-resistors. Such bulk micromachined devices are large by integrated circuit standards and consistent with semiconductor circuit fabrication techniques.
Another system made by surface micromachining is based on a differential capacitor. Surface micromachining creates much smaller, more intricate and precisely patterned structures than those made by bulk micromachining. It involves the same process that is used to make integrated circuits, namely, depositing and etching multiple thin films and layers of silicon and silicon-oxide to form complex mechanical structures. In this case a central beam is affixed in an xe2x80x9cHxe2x80x9d configuration with the spaced apart parallel arms of the xe2x80x9cHxe2x80x9d supporting respective ends of the cross beam.
A plate affixed perpendicular to the beam forms a moving capacitor plate that is positioned between two fixed plates, thus, forming two capacitors sharing a common moving plate. When the unit is subjected to an accelerating force the beam and hence moving plate moves closer to one of the fixed plates and away from the other fixed plate. The effect is to reduce one of the capacitors and increase the other by an amount proportional to the acceleration. The device requires proper orientation with the cross beam parallel to the direction of acceleration. However, surface micromachining is used to create a much smaller device adapted to the same techniques used to make integrated circuits. The moving capacitor plate accelerometer suffers from high noise and exhibits drift at low acceleration measurements.
It is an object of the present invention to provide an improved accelerometer. It is a further object of the invention to provide an accelerometer having no proof mass and a corresponding increase in ruggedness.
According to the invention there is provided an accelerometer having a substrate with an open space therein, a primary heater and a pair of temperature sensitive elements extending across the open space with the temperature sensitive elements on either side of the primary heater and each spaced 75 to 400 microns from the heater. A non-solid heat transfer medium surrounds the heater and temperature sensitive elements.
Applicant""s prior application Ser. No. 08/673,733 described an accelerometer with a central heater and a temperature sensitive element on either side of the heater with each temperature sensitive element spaced 20 microns away from the heater. However, it has been found that greatly improved sensitivity can be realized by increasing this spacing.
The temperature sensitive elements may be parallel to and equidistant from and located on opposite sides of the primary heater.
An electrical conductor is preferably connectable to an external source of power operative to conduct electric current through the primary heater so as to develop a symmetrical temperature gradient extending outwardly from the primary heater on either side thereof.
A pair of auxiliary heaters may be symmetrically disposed on either side of and spaced from the primary heater.
Advantageously, conductive lines are coupled to the auxiliary heaters and to an external source of power and are operative to permit independent changing of current through each of the auxiliary heaters.
The temperature sensing elements may be thermopiles arranged linearly and substantially parallel to a first direction and located at two positions equidistant from and on either side of the primary heater. Each one of the thermopiles may be operative to produce an electrical potential proportional to the temperature at one of the two positions.
Each of the thermopiles may be comprised of a plurality of thermocouple with each of the thermocouples being made out of a first material and a second material, which form a thermocouple junction in a location where the first and second material are joined. Each of the thermocouple junctions may be operative to produce an electrical potential proportional to the temperature at the thermocouple junction. The plurality of thermocouple junctions may be physically arranged in a linear pattern and electrically coupled in series so as to form an array of thermocouple junctions.